Introduction : Yawning is an
infrequently occurring behavior and is therefore
difficult to study. Our laboratory has produced
two groups of SpragueÐDawley
rats selected for high- (HY) and low-yawning
(LY) frequency (Urba-Holmgren
et al.1990). In previous studies,
differences in yawning between HYand LY have
been attributed to a possible alteration of the
cholinergic or dopaminergic neurotransmission
systems (Urba-Holmgren
et al. 1990, 1993). Novelty-induced grooming
is often higher in HY than in LY rats (Eguibar
and Moyaho,1997), but little is known yet
about the neurochemical mechanisms underlying
grooming differences between both groups of rats
(Eguibar and
Moyaho 1997).

Yawning and grooming are apparently related
behaviors since they tend to occur in
stress-related contexts (Eguibar and Moyaho1997;
Moyaho and Valencia 2002). In previous studies,
it was shown that dopamine D1 receptors are
involved in grooming (Molloy andWaddington 1987;
Van Wimersma Greidanus et al. 1989; Drago et al.
1999), whereas D2 autoreceptors (Mogilnicka
and Klimek 1977; Yamada
and Furukawa 1980; Urba-Holmgren
et al. 1982; Dourish and Cooper 1985) or D2
postsynaptic receptors (Morelli
et al., 1986; Serra
et al., 1986; Scheel-Kruger, 1986; Stahle
1992) bring about yawning. The
circumstancethat HYand LY rats differ in two
behaviors, which distinguish between D1 and D2
receptors, led us to investigate to what extent
dopamine neurotransmission accounts for the
concomitant presence of high numbers of yawns
and grooms in HY rats. Specifically, we tested
whether HY were more responsive than LY rats to
the effect of SKF 38393, a D1 agonist (Setler et
al. 1978), and quinpirole, a D2 agonist (Tsuruta
et al. 1981).We also coadministered single doses
of both drugs to test whether the facilitatory
role that the stimulation of dopamine D1
receptors exerts on the behavioral expression
ofD2 (Longoni et al. 1987) could explain the
difference between the two groups of rats.
[...]

Discussion : Although we applied
consecutive doses of quinpirole andSKF 38393 to
the same subjects, accumulative effects on the
measured behaviors are unlikely since 48 h
elapsed between doses. In addition, it is known
that SKF 38393 does not generate behavioral
tolerance (Neisewander et al., 1991). Moreover,
the doseÐresponse curves of yawning and
grooming do not fit the prediction that greater
doses would be necessary to reinstate the
initial effect (Carlton, 1983).The increment in
the number and duration of grooming episodes
with SKF 38393 in HY and LY rats is consistent
with previous studies on other strains of rats
(Braun andChase, 1987; Longoni et al., 1987;
Molloy and Waddington,1987; Serra et al., 1990),
and accords with the view that D1 receptor
activation contributes to the initiation as well
as the completion of grooming sequences
(Berridge and Aldridge,2000).

The finding that HY and LY rats did not
differ in the number or duration of grooming
episodes in response to SKF38393 suggests that
dopamine D1 receptors do not play a direct
role in grooming between these groups of
rats. Incontrast, the fact that quinpirole did
not inhibit grooming episodes in HY rats as much
as in LY rats suggests that D2 receptor
activation can produce differences in
grooming between the two groups of rats.
Since the difference was restricted to the
frequency of grooming episodes, it appears that
quinpirole can distinguish between grooming
frequency and duration.

Although the decrease in grooming episodes
with quinpirole is consistent with previous
studies (White etal., 1988; Eilam and Szechtman,
1989; Eilam et al., 1989,1992; Jackson et al.,
1989, but see Braun and Chase, 1987;Walters et
al., 1987), there are no previous reports, as
far aswe know, in which quinpirole can affect
differentially the frequency and duration of
grooming episodes.The increment in
yawning with quinpirole agrees with previous
reports (Longoni et al., 1987; Spina et al.,
1989;Kostrzewa and Brus, 1991). Similarly, the
direction of the difference in yawning between
HY and LY rats after the administration of
quinpirole accords with studies that indicate
that apomorphine and ( -)-3-PPP have a greater
effect on HY than on LY rats (Urba-Holmgren et
al. 1993). In thepresent study, HY and LY rats
differed in yawning inresponse to low doses of
quinpirole which primarily affect DA
autoreceptors (Di Chiara et al., 1978). This
agrees with the suggestion that yawning is an
autoreceptor-mediated response, although D1
receptor activation may also be involved, as the
inhibitory effect of SKF 38393 on yawning
differed between HY and LY rats. In fact other
studies havequeried the hypothesis that yawning
is an autoreceptor mediated response (Stahle and
Ungerstedt, 1986), and some authors have
provided suggestive evidence for a role of
dopamine D3 receptors in eliciting yawning
(Kostrzewa and Brus, 1991; Damsma et al., 1993).
Therefore, there is the possibility that
differences in yawning frequency between HY and
LY rats are the result of quinpirole activating
D3 receptors.

On the other hand, there is evidence that
dopamine D1 and D2 receptors are coupled (Arnt,
1985a,b), and that the stimulation of D1
receptors by endogenous dopamine exerts a
facilitatory role in the behavioral expression
of D2 receptor activation (Morelli et al., 1986;
Braun and Chase, 1987;Longoni et al., 1987).
According to this hypothesis, the
coadministration of quinpirole and SKF 38393
would potentiate yawning. However, the results
did not corroborate this prediction, as yawning
was below the level reached when quinpirole
alone was administered. Nor did the results
agree with earlier studies which indicate that
D1 receptor activation inhibits yawning induced
by apomorphine (Zarkovsky andCereska, 1989) or
by bromocriptine (Canales and Iversen,2000), a
D2-class receptor agonist. Instead, our findings
indicate that when coadministered, the effect of
quinpirole acted against SKF 38393.In summary,
the results presented here indicate that the
activation of D2 receptors contributes to a
greater extentt han is the case for D1 receptors
to yawning and grooming differences between HY
and LY rats.